There are several low-power cryogenic electronic technologies that require broadband communication of analog or digital data to conventional electronic systems at room temperature. These include ultrafast superconducting circuits for data conversion, radio communications, and computing, as well as superconducting circuits for quantum computing and sensitive imaging arrays across the electromagnetic spectrum. A key problem with these systems is that the characteristic voltage output of these low-power systems is 1 mV or even less, while room-temperature semiconductor electronic devices have a typical voltage level of order 1 V or more. In some cases, one can use a high-gain transistor amplifier to bridge this gap, but such transistor amplifiers may dissipate too much power for the cryogenic environment. Furthermore, the electrically conductive high-bandwidth transmission output lines themselves may conduct too much heat to the cryogenic environment. (See FIG. 1A.) See, for example, Wuensch et al., “Cryogenic Semiconductor Amplifier for RSFQ Circuits with High Data Rates at 4K”, IEEE Transactions on Applied Superconductivity, vol. 19, pp. 574-579, 2009; Gupta et al., “Low-power High-Speed Hybrid Temperature Heterogeneous Technology Digital Data Links”, IEEE Transactions on Applied Superconductivity, vol. 23, article 1701806, 2013.
The key issues for such devices include how much power dissipation is required in or near the cryocooler which maintains the superconductive electronics at operating temperature, how much heat transfer occurs into the cryocooler through the transmission lines, and how well the transmission lines perform. Ancillary issues relate to how large and/or complex are, and what type of circuits within the cryocooler are required to drive the transmission lines.
A promising alternative to electrical transmission lines is optical communication via optical fibers, which can be constructed to have high performance while conducting very little heat. Note that these optical transmission lines need not be very long, and therefore the optical signal may be weak as compared to long distance room temperature telecommunication fibers. This requires an electro-optic transducer at the low-temperature end, e.g., within the cryocooler or at an intermediate temperature, of which several types are well known in the prior art. These include several types of small semiconductor lasers (such as vertical cavity surface emitting lasers or VCSELs) and light-emitting diodes (LEDs). (See FIG. 1B.) See, for example, Mukhanov et al., “Development of Energy Efficient Cryogenic Optical Data Link”, Proc. Superconductive Electronics Conference, 2013; Bunz et al., “Laser Diodes for Optical Readout of Cryoelectronics”, Proc. SPIE 2226, Infrared Readout Electronics II, 50 (Jun. 23, 1994).
Alternatively, one may send an optical fiber with a carrier optical wave down to the cryogenic environment, and subject this carrier wave to modulation in an electro-optic modulator device. Several types of modulator devices are known in the prior art, but most of these are semiconductor devices that require an input voltage of order 1 V or more, and may also dissipate significant levels of power. Again, one could use a transistor amplifier to bridge the voltage gap, with the same power limitations as above. (See FIG. 1C.) See U.S. Pat. Nos. 4,810,978; 6,661,560; 5,210,637. Note that carrier mobility, may be low at cryogenic temperatures, and thus semiconductor technology amplifiers may have impaired performance.
See “Direct drive optical link for high-speed cryogenic data readout”, www.sbir.gov/sbirsearch/detail/377697, Navy ID N11A-022-0400; Marek Osinski and Gennady A. Smolyakov; “Injection locked microring lasers for ultrafast modulation”, spie.org/x84982.xml.
The prior art does not provide an optical source or modulator that is sensitive to an input signal of 1 mV or less, which dissipates very little heat and requires little or no input amplification to produce a detectable signal through an optical fiber of about 5-250 cm.
Very recently, electro-optic modulators based on the unique material graphene have been reported. Graphene comprises a single atomic layer of a hexagonal lattice of carbon atoms (though multiple layers may be provided in some cases), and can be viewed as a zero-gap semiconductor with extremely high electron mobility. (In multiple layer structures, electron mobility is anisotropic). Graphene is also a strong optical absorber over a broad optical band, which is remarkable given its single atomic layer. Together, these enable very small optoelectronic devices with low levels of power dissipation.
For example, U.S. Pat. No. 9,360,689, and U.S. patent application Ser. No. 2014/0056551 (Liu et al), “Graphene Based Optical Modulator”, (see FIG. 2) provides a modulator which comprises either one or two graphene monolayers embedded in an optical waveguide. See also,
U.S. Pat. No. 8,983,251 (Lu et al), “Electro-optical waveguide apparatus and methods thereof”;
U.S. Pat. No. 9,042,283 (Cho et al.), “Optical modulator including graphene”;
Jiaqi Wang, Zhenzhou Cheng, Zefeng Chen, Jian-Bin Xu, Hon Ki Tsang, and Chester Shu, “Graphene photodetector integrated on silicon nitride waveguide” J. Applied Physics 117, 144504 (2015);
Nathan Youngblood, Yoska Anugrah, Rui Ma, Steven J. Koester, and Mo Li, “Multifunctional Graphene Optical Modulator and Photodetector Integrated on Silicon Waveguides”, dx.doi.org/10.1021/nl500712u, Nano Lett. 2014, 14, 2741-2746;
Steven J. Koester, Huan Li, and Mo Li, “Switching energy limits of waveguide-coupled graphene-on-graphene optical modulators”, Optics Express, Vol. 20, No. 18, 20330 (27 Aug. 2012);
Nicholas Hunter, Alexander S. Mayorov, Christopher D. Wood, Christopher Russell, Lianhe Li, Edmund H. Linfield, A. Giles Davies, and John E. Cunningham, “On-Chip Picosecond Pulse Detection and Generation Using Graphene Photoconductive Switches”, DOI: 10.1021/nl504116w, Nano Lett. 2015, 15, 1591-1596;
Luo et al. Nanoscale Research Letters (2015) 10:199, DOI 10.1186/s11671-015-0866-7; Muhammad Mohsin, Daniel Neumaier, Daniel Schall, Martin Otto, Christopher Matheisen, Anna Lena, Giesecke, Abhay A. Sagade & Heinrich Kurz, “Experimental verification of electro-refractive phase modulation in graphene”, Scientific Reports 5, Article number: 10967 (2015), doi:10.1038/srep10967;
Liu, Ming, Xiaobo Yin, and Xiang Zhang. “Double-layer graphene optical modulator.” Nano letters 12.3 (2012): 1482-1485;
Liu, Ming, et al. “Graphene optical modulator.” SPIE NanoScience+Engineering. International Society for Optics and Photonics, 2011; Midrio, Michele, et al. “Graphene-assisted critically-coupled optical ring modulator.” Optics express 20.21 (2012): 23144-23155;
Xu, Chao, et al. “Characteristics of electro-refractive modulating based on Graphene-Oxide-Silicon waveguide.” Optics express 20.20 (2012): 22398-22405;
Locatelli, Andrea, et al. “Graphene-assisted control of coupling between optical waveguides.” Optics express 20.27 (2012): 28479-28484;
Gosciniak, Jacek, and Dawn TH Tan. “Theoretical investigation of graphene-based photonic modulators.” Scientific reports 3 (2013);
Midrio, Michele, et al. “Graphene-based optical phase modulation of waveguide transverse electric modes.” Photonics Research 2.3 (2014): A34-A40;
Gosciniak, Jacek, and Dawn TH Tan. “Graphene-based waveguide integrated dielectric-loaded plasmonic electro-absorption modulators.” Nanotechnology 24.18 (2013): 185202;
Mohsin, Muhammad, et al. “Graphene based low insertion loss electro-absorption modulator on SOI waveguide.” Optics express 22.12 (2014): 15292-15297;
Ryzhii, Victor, et al. “Double-graphene-layer terahertz laser: concept, characteristics, and comparison.” Optics express 21.25 (2013): 31567-31577;
Tamagnone, Michele, et al. “Fundamental limits and near-optimal design of graphene modulators and non-reciprocal devices.” Nature photonics 8.7 (2014): 556-563;
Koester, Steven J., and Mo Li. “Waveguide-coupled graphene optoelectronics.” Selected Topics in Quantum Electronics, IEEE Journal of 20.1 (2014): 84-94;
Ye, Shengwei, et al. “Electro-absorption optical modulator using dual-graphene-on-graphene configuration.” Optics express 22.21 (2014): 26173-26180;
Cho, Seong-Ho, and Hyun-jong Chung. “Optical modulator including graphene.”
Chen, Pai-Yen, et al. “Graphene-based plasmonic platform for reconfigurable terahertz nanodevices.” ACS Photonics 1.8 (2014): 647-654;
Kwon, Min-Suk. “Discussion of the epsilon-near-zero effect of graphene in a horizontal slot waveguide.” Photonics Journal, IEEE 6.3 (2014): 1-9;
Phare et al, “Graphene Electro-Optic Modulator with 30 GHz Bandwidth”, Nature Photonics, vol. 9, pp. 511-514, August 2015 (See FIG. 3), shows a high-Q optical resonator, of which the graphene layer is a part. Applying a voltage tunes the Fermi energy of the graphene via the field effect, which in turn affects the optical absorption in the graphene, and changes the Q and hence the impedance of the resonator, thus shifting the resonance slightly and modulating the transmitted optical wave.
All of these disclose devices designed to operate at room temperature, with no cryogenic properties disclosed or predicted. Some of these devices are somewhat sensitive to small voltages, and further have other important advantages, such as compact size and broad optical bandwidth.
There are several recent reports of graphene devices operating at cryogenic temperatures, including hybrid devices with superconducting materials. See, for example, see:
McKitterick et al., “Graphene Microbolometers with Superconducting Contacts for Terahertz Photon Detection”, published online July 2013 at arxiv.org/abs/1307.5012;
Calado et al., “Ballistic Josephson Junctions in Edge-Contacted Graphene”, published online June 2015 at arxiv.org/abs/1501.06817;
Chapman et al., “Superconductivity in Ca-Doped Graphene”, published online August 2015 at arxiv.org/abs/1508.06931;
Weber et al., “Coupling graphene mechanical resonators to superconducting microwave cavities, published online April 2014 at http://arxiv.org/abs/1403.4792;
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